MULTIFREQUENCY ACOUSTIC CHAMBER FOR THE AGGLOMERATION AND SEPARATION OF SUSPENDED PARTICLES IN GAZ EFFLUENTS

Abstract: The object of the present invention is an acoustic chamber for the treatment, at various frequencies and high intensities, of gas effluents which contain solid and liquid microparticles in suspension. The acoustic energy agglomerates the microparticles, thereby facilitating the air separation by means of conventional ystems (electro-filters, cyclones, etc.). The chamber has a rectangular cross section and is axially traversed by the flow of aerosols to be agglomerated (1) while the acoustic generators which are of the stepped vibrating plate type (2) and which may have different frequencies, are contiguously of alternatingly arranged on the side walls in order to create intensive stationary fields in the various cross sections of the chamber. In order to maximize the emission of both faces of the radiant plate of the acoustic generators, it is proposed to arrange the plate between two forming angle planar reflectors. Additionally, for the treatment of aerosols at high temperatures, there is provided a refrigeration system (4) in order to refrigerate the acoustic radiators by means of a cool air flow.

Description: Particles suspended in gases coming from industrial emissions or from the exhausts of internal combustion engines constitute one of the factors with the greatest impact on atmospheric contamination. More specifically, the very fine particles (smaller than 5 micrometers) which are very difficult to eliminate with conventional separation technologies represent a major heath hazard because of their penetration and adhesion ability to respiratory tissue as well as for their general toxicity. At this time, efficient industrial emission control of particulate matter is only possible down to particle sizes in the order of some microns by means of electrostatic filters. Acoustic energy offers a new method for elimination of micrometer- and submicrometer-sized particles. The Spanish Patent 439.523 indicates how the application of a high intensity acoustic field with an appropriate frequency in an aerosol gives raise to an agglomeration processes of the particles, which form the aerosol, increasing their size to facilitate in this way their later precipitation or separation in a conventional or gravity collector. The process takes advantage of the effects of vibration, hydrodynamic interaction, and entrainment which are produced between the particles by the action of the acoustic field. The invention object of the present patent refers to an acoustic chamber for treatment of gaseous effluents which contain solid or liquid particles in suspension. There are some precedents of equipment of this type. The Spanish Patent 459.523 presents a cylindrical chamber of which one end incorporates an acoustic emitter of radiating-stepped-plate type and the other one a plane reflector paralleling the face side of the emitter. The aerosol to be treated is introduced tangentially and it carries out a helicoidal course which, even though favoring the particle agglomeration, introduces load losses and provokes turbulent flow. EP-A-488097 presents a new chamber type of polygonal cross section in which the aerosol enters axially while the acoustic emitter or emitters (whose type is not specified) are located at an oblique angle with respect to the chamber axis. This way, the acoustic radiation is repetitively reflected on the chamber walls until it falls, on the other end of the chamber, onto a plane reflector which is also located at an oblique position with respect to the axis. This oblique incidence, however, can favor the excitation of multiple modes which can produce energy dispersion between the principal axial mode and the different transversal modes. On the other hand, with the use of several acoustic sources of equal frequency within the same chamber section one risks to produce negative interferences by generating the acoustic field. Both inventions try to take advantage of the maximum axial length of the chamber and of the application of a single operating frequency. Cooling devices for the acoustic emitters, which permit the operation with gases at high temperatures, are proposed in none of these systems. DE-A-23 04 597 discloses an arrangement for agglomeration of particles in gaseous effluents by means of acoustic waves. The arrangement comprises a chamber which includes to opposite walls with mutually parallel surfaces; on one of said walls, an acoustic emitter is situated, which emits acoustic radiation in a direction perpendicular to said wall. The radiation is reflected against the opposite wall, giving rise to a standing wave. The object of the present invention as claimed is a new chamber type with polygonal, preferably rectangular cross section based on the application of high acoustic energy levels by means of radiating-stepped-plate type generators (cf. EP-A-450030) which treat an aerosol flow, throughout its passing-by, in direction perpendicular to the propagation (Figure 1). In this manner, the gases to be treated will flow in a straight line along the chamber axis, indicated by Arrow 1, while the emitters (2), which are of different frequencies, will be distributed on the side walls to create intensive stationary fields which will cover the treatment chamber from end to end in all its different sections without leaving extensive zones at low acoustic levels. The chamber walls opposing the emitters will function as reflectors. The transducer support implements an adjustment device (3) which permits the variation of the radiator-reflector distance to achieve an optimum stationary field. By letting every acoustic emitter act on the width of the chamber (instead of on the length, as done in the former mentioned inventions) one will need to cover smaller distances and so one will be able to achieve a higher intensity as well as a better homogeneity of the acoustic field, since generally one will work in the nearfield of the emission where the beam maintains practically parallel. The application of different frequencies along the flow path is very important since, because of the existence of an optimum agglomeration frequency for each particle size, the change of the size distribution through the process itself requires a change of the treatment frequency. In Figure 1, where following the present invention a scheme of the acoustic chamber for aerosol treatment is presented, it is also proposed to include a cooling system for the transducers for their use in treatment of high temperature gases. This system is based on the introduction of a cold or ambient temperature air flow (at the level of the radiating plate) which acts as a gaseous curtain separating the acoustic emitter and the high temperature aerosol. This way, a temperature gradient is produced which not only serves to thermally protect the emitter but also, in addition, to produce a gradual effect of impedance adaptation between the radiator and the medium. Such a system might be required for those industrial applications in which the gaseous effluents proceeding combustion processes are emitted at temperatures in the order of 150°C or even higher. As said before, the acoustic emitters to be used in the proposed chamber are of type "plate radiator with stepped profile." In the arrangement shown in Figure 1, the emission from the plate's backside is not taken advantage of for the agglomeration process, except in the case if the mentioned plate is completely introduced in the interior of the treatment chamber. An arrangement with which the mentioned backside radiation could be realized is presented in Figure 2. The transducer (2) is located with its axis parallel to that of the chamber in a way that the radiating plate is situated in between of two reflector plates (5), which form an angle such that the forward as well as the backward radiation can be reflected and normally directed into the gas stream which is flowing along the treatment chamber (Figure 3). Like this, one obtains an efficient use of the total radiated energy of the emitter. It is to say, however, that this arrangement implies major difficulties on the adjustment and realization of an optimum stationary wave. As an example, we can show that with 150 W power a 20 kHz transducer, located with its axis parallel to that of the chamber in between of two plane reflectors which form a 90° angle and are positioned at a 45° angle with respect to the radiating plate, achieves, in a chamber with a cross section of 140 x 60 cm, acoustic pressure levels higher than the 150 dB which are necessary for rapid generation of the acoustic agglomeration phenomenon. Without taking advantage of the backside radiation by means of reflectors, the mentioned pressure levels are reached with applied powers in the order of 200 W. In a rectangular chamber with a cross section of 0,5 x 0,5 m and a length of about 2 m it is possible to treat aerosol flows of from 1000 to 2000 m 3 /h by applying four radiating plate transducers with diameters of 48 cm and radiated acoustic powers in the order of 300 W/unit. The transducers are located as groups along one of the chamber walls or alternatingly as shown in Figure 1. If the backside radiation is not taking advantage of by abstaining from using angled reflectors with the radiators, the effective acoustic potential is approximately one half of the radiated one, that is to say in the order of 150 W/unit. This means that the total power applied to the aerosol on its run throughout the treatment chamber is about 600 W. The intensity levels achieved in the interior of the chamber are higher than 160 dB. This way, one achieves, with frequencies of 20 kHz for aerosols with initial particles from 0,2 to 2 micrometers and concentrations from 0,1 to 4 g/m 3 , enlargement of an order of magnitude. That is to say, the mean size grows from one to ten micrometers. This increase is decisive for the subsequent application of an electrostatic filter to the aerosol flow, since these filters are generally efficient from 5 micrometers on.

Claims: Multifrequency acoustic chamber for agglomeration and separation of particles suspended in gaseous effluents, the chamber having a polygonal cross section and each wall facing an opposite wall parallel therewith, such that acoustic radiation emitted orthogonally in a direction from one wall will fall orthogonolly on a surface of an opposite wall and be reflected by said opposite wall, giving rise to the generation of a stationary acoustic field; said chamber having an input end and an output end, the walls of the chamber defining a flow path in a longitudinal direction from said input end to said output end; the chamber including a plurality of acoustic emitters (2), said emitters being disposed at said walls at positions along said flow path, each of said acoustic emitters (2) being arranged to emit radiation of high intensity at a determined frequency in an orthogonal direction with respect to the wall at which said acoustic emitter (2) is placed; said acoustic emitters being disposed to emit directly from a front side of a radiator plate into said flow path; characterized in that said acoustic emitters (2) are of the radiating-stepped-plate type; and in that the acoustic emitters (2) are arranged such that radiation of different frequencies is emitted at different longitudinal positions along said flow path, in order to provide an adequate frecuency distribution in view of the particle size distribution along said longitudinal flow path. Multifrequency acoustic chamber for agglomeration and separation of particles suspended in gaseous effluents, the chamber having a polygonal cross section and each wall facing an opposite wall parallel therewith, such that acoustic radiation emitted orthogonally in a direction from one wall will fall orthogonolly on a surface of an opposite wall and be reflected by said opposite wall, giving rise to the generation of a stationary acoustic field; said chamber having an input end and an output end, the walls of the chamber defining a flow path in a longitudinal direction from said input end to said output end; the chamber including a plurality of acoustic emitters (2), said emitters being disposed at said walls at positions along said flow path, each of said acoustic emitters (2) being arranged to emit radiation of high intensity at a determined frequency in an orthogonal direction with respect to the wall at which said acoustic emitter (2) is placed; characterized in that said acoustic emitters (2) are of the radiating-stepped-plate type; and in that the acoustic emitters (2) are disposed to emit into the flow path by means of two plane reflectors (5) located on opposite sides of a radiator plate of the acoustic emitter (2) to form an angle with said radiator plate in such a way that radiation from a front side and a back side of said radiator plate is reflected by said plane reflectors and directed into said flow path in an orthogonal direction with respect to the wall at which the acoustic emitter (2) is placed; and in that the acoustic emitters (2) are arranged such that radiation of different frequencies is emitted at different longitudinal positions along said flow path, in order to provide an adequate frecuency distribution in view of the particle size distribution along said longitudinal flow path. A multifrequency acoustic chamber according to claim 1, wherein the acoustic emitters (2) are provided with a cooling system including means for providing a laminar flow of cool air which serves as a curtain separating the acoustic emitter from hot gaseous effluents passing along the flow path. A multifrequency acoustic chamber according to any of the preceding claims, wherein the acoustic emitters (2) are placed along the chamber walls as contiguous units or groups. A multifrequency acoustic unit according to any of claims 1-3, wherein the acoustic emitters (2) are placed on alternating walls. A multifrequency acoustic chamber according to any of the preceding claims, wherein the chamber is provided with adjustment systems (3) for adjusting the position of said radiator plates with respect to reflecting surfaces. A multifrequency acoustic chamber according to any of the preceding claims, wherein said polygonal transverse cross section is a rectangular transverse cross section.

Inventors: GALLEGO JUAREZ JUAN A, RIERA FRANCO DE SARABIA, RODRIGUEZ CORRAL, GALLEGO JUAREZ, JUAN A., GALLEGO JUAREZ, Juan A., Inst. Acustica Consejo, RIERA FRANCO DE SARABIA, Germán, Inst. Acustica, RODRIGUEZ CORRAL, Germán, Inst. Acustica
Applicants: CONSEJO SUPERIOR INVESTIGACION, CONSEJO SUPERIOR INVESTIGACIONES CIENTIFICAS (CSIC)
Application year: 1994
Main classification: B01D49
IPC codes: B01J19, B01D49